Organic, Polymer, Materials Chemistry, Nanocomposites, Self-Assembly, Energy Storage & Conversion, Sulfur Utilization, Sulfur Chemistry & Processing, Batteries, Semiconductor nanoparticles, Heterostructured Nanorods, Photocatalysis
Our research program is focused on the synthesis and characterization of novel polymeric and composite materials, with an emphasis on the control of nanoscale structure. Recent developments in polymer and colloid chemistry offer the synthetic chemist a wide range of tools to prepare well-defined, highly functional building blocks. We seek to synthesize complex materials from a "bottom up" approach via the organization of molecules, polymers and nanoparticles into ordered assemblies.
Control of structure on the molecular, nano- and macroscopic regimes offers the possibility of designing specific properties into materials that are otherwise inaccessible. We are particularly interested in compatabilizing interfaces between organic and inorganic matter as a route to combine the advantageous properties of both components. This research is highly interdisciplinary bridging the areas of physics, engineering and materials science with creative synthetic chemistry.
We are using these synthetic techniques & processing methods to prepare nanomaterials for applications in energy storage & conversion, focusing on photocatalytic water splitting and next generation batteries.
High Sulfur Content Polymers for Energy Storage and Optics
Efforts in the Pyun Group are focused on the development of new chemistry to directly utilize elemental sulfur as a cheap feedstock for polymers and nanomaterials.
Synthesis and Imaging of Heterostructured Nanomaterials (II-VI Semiconductor quantum dots, nanorods, tetrapods, etc.) and Colloidal Copolymers
In recent years, interest in ensemble properties of colloidal nanoparticles has led to the development of new systems with properties of self-assembly. These assemblies of inorganic nanomaterials, termed colloidal polymers, have been shown to exhibit properties (optical, electronic, magnetic, catalytic) distinct from their discreet components (colloidal monomers), which can be tuned as a function of extent of assembly (degree of polymerization). To this end, akin to traditional polymer systems, selective tuning of monomer functionality, assembly motif, and polymer architecture for colloidal polymer systems is of key interest to exploit the unique properties of new monomer units.
Recently we have demonstrated the design of new colloidal monomers, consisting of a II-VI semiconductor (SC) functional group carrying a single gold nanoparticle (AuNP) as a polymerizable unit. Through a simple, one pot reaction, dipolar cobalt shells can be grown selectively on the AuNP tips, resulting in magnetic associations and effective polymerization into linear structures. We have demonstrated the ability to the tune SC pendant group, from nanorod (NR) to tetrapod (TP) structures, allowing for clear visualization of polymer composition through TEM. Further, in addition to homopolymer systems from these units, blending of NR and TP monomers with AuNP tips of identical size, followed by dipolar cobalt deposition allowed access to statistical copolymers. Intriguingly, by varying the size of AuNP tip (and thus Au/Co tip size), monomer reactivity could be tuned, with smaller particles leading to weaker dipolar interactions. This discovery allowed for the first ever analogue of reactivity ratio control for colloidal polymer systems, wherein blending of NR and TP monomers with drastically different AuNP tip sizes prior to polymerization afforded segmented copolymers. For traditional copolymer systems, reactivity ratios are typically monitored by comonomer kinetics. For the first time with these colloidal copolymers, we can tune reactivity ratios controllably, and directly correlate reactivity ratios with polymer composition that can be intuitively visualized by TEM, unlike classical polymer systems.
[FeFe]-Hydrogenase Mimetic Metallopolymers for H2 Generation in Water
Metal-containing polymers can combine the useful properties of polymers with the key functions of metal complexes. These metallopolymers are applicable to a wide range of areas such as photovoltaics, stimuli responsive materials and catalysis. Catalysis, for example, includes designing artificial metalloenzymes which can mimic the biological functionalities by engineering the environment of a metal complex using polymeric materials. FeFe-hydrogenase enzyme found in bacteria is an efficient H2 generation catalyst and there has been extensive research on making FeFe-H2ase mimics to produce H2 as a carbon-free energy carrier. The mimics have shown high catalytic activities in organic media, however, limited lifetime, low oxygen stability and low solubility preclude the applicability of the mimics. We, for the first time, made a metalloinitiator from a FeFe-H2ase mimic to grow polymers via ATRP. The polymers not only provide water solubility and oxygen stability in neutral water but also enhance the activity of the complex by tuning the secondary coordination sphere of the mimic. We will discuss our most recent efforts to synthesize a difunctional metalloinitiator and metallopolymers grafted via ATRP.
Polymers and Ferromagnetic Nanoparticles: Novel Building Blocks for Dipolar Assemblies
We are pursuing a modular synthesis of nanocomposite chains composed of ferromagnetic colloids and functional block copolymers. Controlled and living polymerizations allow the organic chemist to prepare of a wide range of functional block copolymers which will be used as polymeric surfactants in the formation of magnetic colloidal dispersions. In the presence of a magnetic field, these core-shell magnetic nanoparticles align into polymeric chains. The assembled chain can then be locked in by crosslinking of reactive groups attached to the block copolymer surfactant. It is anticipated that the hybridization of these components on the nanoscale will synergistically combine the beneficial film forming properties of organic polymers with the magnetic character of the inorganic colloid. We have developed new synthetic methods using polymeric surfactants to prepare well-defined ferromagnetic nanoparticles and demonstrated that dispersed colloids can be magnetically assembled into mesoscopic 1-D nanoparticle chains, which we refer to as mesoscopic polymer chains, or “meso-polymers.” Functionalization and controlled assembly of these novel building blocks are currently being pursued.
Funding: National Science Foundation, Office of Naval Research, American Chemical Society-Petroleum Research Fund
Collaborators: Jason Benkoski (Johns Hopkins Applied Physics Lab), Jack Douglas (NIST), Inbo Shim (Kookmin University, Seoul, Korea)
Colloidal Polymerization of Dipolar Cobalt Nanoparticles and Heterostructured Colloidal Monomers
Ferromagnetic nanoparticles are intriguiging self-assembling colloids that organize into 1-D nanoparticle chains via spin dipolar interactions. We have developed a novel synthetic methodology, termed, “Colloidal Polymerization” where ferromagnetic polystyrene coated cobalt nanoparticles were used as “colloidal monomers” to form fused cobalt oxide nanowires. We have demonstrated that these cobalt oxide nanowires are electroactive and intriguing materials for electrochemical energy conversion and catalysis. To enhance the properties of these cobalt oxide nanowires, we have achieved using combinations of dipolar assembly and nanoparticle conversion chemistry (e.g., oxidation, galvanic exchange) to prepare cobalt oxide nanowires with controlled placement of noble metal and semiconductor inclusions. Incorporation of these metals and semiconductor inclusions have been demonstrated to create novel heterojunctions which enhance the electrical, (photo)electrochemical and catalytic properties of the cobalt oxide phase. More recently, we have prepared complex colloidal monomers based on tipping II-VI semiconductor nanorods with noble and magnetic metal NP peripheral inclusions which enable the creation of p-n type junctions composed of p-type oxides with n-type chalcogenides.
Funding: Department of Energy-Basic Energy Sciences
Collaborators: Neal Armstrong, Scott Saavedra (U of A), Kookheon Char, Yung-Eun Sung (Seoul National University), Nicola Pinna (Humboldt University-Germany)
Elemental Sulfur: A novel, abundant feedstock for polymers and nanocomposite materials
We have recently explored the utilization of elemental sulfur for novel polymers and nanocomposites. Sulfur is commonly used as a vulcanizing agent in the crosslinking of rubber for tires, however the use of elemental sulfur as the primary monomer, or comonomer for polymeric materials has not been widely explored. Elemental sulfur is currently produced on the level of 70 million tons annually, the majority of which is thru hydrodesulfurization of crude petroleum. Consequently, over 6 million tons of elemental sulfur is generated in excess, which creates exciting opportunities to develop new chemistry and processing to utilize sulfur as a feedstock for polymers. Sulfur exhibits a number of useful properties, such as, high charge capacity for Li-insertion electrochemistry and high refractive index. However, the chemical modification of sulfur into useful materials remains a difficult technical challenge. Toward this end, we are developing new polymerization and processing methods for the direct utilization of sulfur to prepare thermosetting polymeric sulfur and nanocomposite materials.
Funding: ACS PRF, University of Arizona
Collaborators: Richard Glass (U of A), Kookheon Char, Yung-Eun Sung (Seoul National University), Patrick Theato (Hamburg University-Germany)
Next generation Li-S batteries
We are exploring the synthesis of novel polymeric and nanocomposite materials as cathode electrodes in Li-S batteries. These systems exhibit specific capacities 4-5 times greater than current Li-ion batteries, but currently suffer from limited cycle lifetimes. We are exploiting our new sulfur chemistry to prepare enhanced electrochemically active materials and within the group, currently examining the fabrication and testing of these materials in Li-S batteries.